Thin film multi-layer high Q transformer formed in a semiconductor substrate

ABSTRACT

A thin-film multi-layer high Q transformer. To form an outer transformer winding a plurality of parallel first level metal runners are formed in a first insulating layer overlying the semiconductor substrate. A plurality of vertical conductive vias are formed in third and fourth insulating layers and in electrical communication with each end of the first level metal runners. A fourth insulating layer is disposed over the third insulating layer and additional vertical conductive vias and a fourth level metal runner are formed therein. Thus, the fourth level metal runners and the intervening vertical conductive vias connect each of the first level metal runners to form a continuously conductive structure having a generally helical shape. The inner winding of the transformer is similarly formed. A plurality of parallel second level metal runners are formed within the second insulating layer and a plurality of conductive vias and third level metal runners are formed within the third insulating layer to interconnect the plurality of second level metal runners to form a continuously conductive structure having a generally helical shape and disposed at least partially within the outer transformer winding.

[0001] This patent application claims priority to the provisional patent application filed on Jun. 28, 2001, and assigned serial No. 60/301,285.

FIELD OF THE INVENTION

[0002] This invention relates generally to transformers formed on an integrated circuit substrate, and more specifically to transformers having an outer core spanning at least three metal layers of the integrated circuit substrate.

BACKGROUND OF THE INVENTION

[0003] The current revolution in wireless communications and the need for smaller wireless communications devices has spawned significant efforts directed to the optimization and miniaturization of radio communications electronics devices. Passive components of these devices (such as inductors, capacitors and transformers), play a necessary role in the devices' operation and thus efforts are directed toward reducing the size and improving the fabrication efficiency of such components.

[0004] Transformers, which play an integral role in the performance of electronic communications devices, are electromagnetic components comprising a primary and a secondary winding. Conventionally, the windings are wound on a common core, which forms a closed loop magnetic circuit. Iron cores are typical to enhance the transformer effect, but not required. Each winding comprises a plurality of turns. The relationship between the primary and secondary voltage is a function of the primary to secondary turns ratio, and the relationship between the primary and secondary current is an inverse function of the turns ratio. As is known, there are many different physical configurations for the transformer windings and core. In one embodiment, for example, the primary and secondary windings form a helical structure, with the secondary windings oriented within the opening formed by the primary windings. Transformers also serve in varied applications, including power applications for stepping applied voltages up or down and for impedance matching at frequencies from audio to radio frequency (RF). With the continual allocation of operational communications frequencies into higher frequency bands, transformers used in impedance-matching applications suffer impaired performance due to increased eddy current and skin effect losses.

[0005] The Q (or quality factor) is an important transformer figure of merit. The Q measures the ratio of inductive reactance to inductive resistance within the transformer windings. High Q transformers present a narrow peak when the transformer current is graphed as a function of the input signal frequency, with the peak representing the frequency at which the transformer resonates. High Q transformers are especially important for use in frequency-dependent circuits operating with narrow bandwidths. Because the Q value is an inverse function of transformer resistance, it is especially important to minimize the resistance to increase the Q.

[0006] Most personal communications devices incorporate integrated circuit active components fabricated using semiconductor technologies, such as silicon or gallium-arsenide. The prior art teaches certain integrated inductive structures (including torroidal or spiral shaped inductors) developed to achieve compatibility with the silicon-based integrated circuit fabrication processes. When two such inductors are proximately formed, the coupling of the magnetic field formed by current flow through one winding (the primary) into the winding area of the other winding (the secondary) results in transformer action and the flow of current in the secondary. However, such planar inductors tend to suffer from high losses and low Q factors at the operative frequencies of interest. These losses and low Q factors are generally attributable to dielectric losses caused by parasitic capacitances and resistive losses due to the use of thin and relatively high resistivity conductors in the transformer structure. Another disadvantage of conventional planar inductors and transformers formed from them is a result of the magnetic field lines (which are perpendicular to the semiconductor substrate surface) entering the semiconductor and dielectric layers above, beside and below the transformer. This increases the inductive losses and lowers the transformer's Q factor. Also, unless the transformer is located a significant distance from active circuit elements formed in the silicon, the magnetic field lines induce currents in and therefore disrupt operation of the active components.

[0007] With integrated circuit active devices growing smaller and operating at higher speeds, the interconnect system should not add processing delays to the device signals. Use of conventional aluminum interconnect metallization restricts circuit operational speed as the longer interconnects and smaller interconnect cross-sections increase the interconnect resistance. Also, the relatively small contact resistance between the aluminum and silicon surfaces creates a significant total resistance as the number of circuit components grows. It is also difficult to deposit aluminum with a high aspect ratio in vias and plugs, where the aspect ratio is defined as the ratio of plug thickness to diameter.

[0008] Given theses disadvantages, copper is becoming the interconnect of choice because it is a better conductor than aluminum (with a resistance of 1.7 micro-ohm cm compared to 3.1 micro-ohm cm for aluminum), is less susceptible to electromigration, can be deposited at lower temperatures (thereby avoiding deleterious effects on the device dopant profiles) and is suitable for use as a plug material in a high aspect ration plug. Copper interconnects can be formed by chemical vapor deposition, sputtering, electroplating and electrolytic plating.

[0009] The damascene process is one technique for forming active device copper interconnects. A trench is formed in a surface dielectric layer and the copper material is then deposited therein. Usually the trench is overfilled, requiring a chemical and mechanical polishing step to re-planarize the surface. This process offers superior dimensional control because it eliminates the dimensional variations introduced in a typical pattern and etch interconnect process. The dual damascene process extends the damascene process, simultaneously forming both the underlying conductive vias and the interconnecting trenches from copper. First the via opening is formed, followed by formation of a trench between two via openings to be interconnected. A subsequent metal deposition step fills both the via openings and the trench, forming an integral metal layer and conductive via to the metal layer below. A chemical and mechanical polishing step planarizes the top surface or the substrate. Dual damascene processes are discussed in detail in the following references, which are hereby incorporated by reference: C. K. Hu et. al., Proceedings MRS Symposium on VLSI, vol. 5, p. 369 (1990); B. Luther et. al. Proceedings VMIC, vol. 10, p. 15 (1994); D. Edelstein, Proceedings ECS Mtg., vol. 96-2, p. 335 (1996).

BRIEF SUMMARY OF THE INVENTION

[0010] To provide further advances in the fabrication of transformers in conjunction with active devices on a semiconductor substrate, an architecture and processes is provided for forming such a transformer within the conventional metal layers of an integrated circuit, wherein the transformer core area is larger than prior art transformers, resulting in a higher inductance value and a higher Q figure of merit. Also, a transformer formed according to the teachings of the present inventions has a desirable low-resistance (and thus high Q) in a relatively compact area of the integrated circuit.

[0011] According to one embodiment of the invention, a plurality of parallel lower conductive strips are formed overlying the semiconductor substrate, in which active components were previously formed. First and second vertical conductive via openings are formed over first and second opposing edges of each lower conductive strip and conductive material is deposited within the via openings to form first and second conductive vias. Two additional via openings are formed in vertical alignment with the first and the second conductive vias and filled with metal to form third and fourth conductive vias. A plurality of upper conductive strips are then formed, wherein the plane of an upper conductive strip intersects the plane of a lower conductive strip such that a first edge of one upper conductive strip overlies the first edge of a lower conductive strip, and the two edges are interconnected by the first and the third conductive vias. A second edge of the upper conductive strip overlies the second edge of the next parallel lower conductive strip, and these edges are electrically connected by the second and the fourth conductive vias. Thus is formed an outer helical winding of the transformer. An inner winding of the transformer is similarly formed. The bottom segment of the inner winding is formed at least one metal layer above the bottom segment of the outer winding, and the top segment of the inner winding is at least one metal layer below the top segment of the inner winding. Although the transformer must be formed in at least four metal layers (i.e., the bottom segment of the inner and windings and the top segment of the inner and outer windings), there can be more than one metal layer between the various winding segments and the bottom segment of the outer winding can be formed on any of the integrated circuit metal layers, with the additional winding segments formed above it.

[0012] The use of certain layout and metallization techniques for constructing a transformer according to the techniques of the present invention result in lower resistive losses in the conductive material, thereby reducing eddy current losses and also increasing the transformer Q factor. According to one embodiment of the present invention, a multi-layer dual-damascene metallization techniques is employed to form the transformer. A plurality of parallel metal-1 runners are formed in a first stack of insulating materials. A second stack of insulating materials is disposed over the first stack and a plurality of first and second via openings are formed therein, wherein each one of the plurality of first via openings is in contact with a first end of a metal-1 runner, and each one of the plurality of second via openings is in contact with a second end of the metal-1 runner. A metal-2 trench is formed within one or more upper layers of the second stack, and the first and the second via openings and trench are then filled with copper. The metal-2 runner is set back from the vertical plane of the metal-1 runner. A third stack of insulating layers is disposed over the second stack and four via openings are formed therein. Third and fourth via openings are each in electrical contact with one of the first and second conductive vias, respectively. Fifth via openings are in contact with a first end of the metal-2 trench and sixth via openings are in contact with a second end of the metal-2 trench. A metal-3 trench is formed interconnecting the upper end of the fifth and sixth via openings, but the metal-3 trench connects two successive metal-2 runners. Thus one end of the metal-3 trench is connected to a sixth via opening of a metal-2 runner and the other end of the metal-3 trench connects with the fifth via opening of the next metal-2 runner in the plurality of parallel metal-2 runners. The third, fourth, fifth and sixth via openings and the metal-3 trenches are then filled with copper. A fourth stack of insulating layers is disposed over the structure and seventh and eighth via openings are formed therein and vertically aligned with the third and fourth conductive vias, respectively. A metal-4 trench is formed in the upper portion of the top insulating stack, with one end of the metal-4 runner in contact with an eighth via opening, and the other end of the metal-4 trench in contact with a seventh via opening of the next metal-1 runner in the plurality of parallel metal-1 runners. In this way, the metal-4 trench interconnects two successive metal-1 runners. The metal-4 trench and the seventh and eighth via openings are filled with copper. In a cross sectional view, the resulting structure forms two concentric rectangles of conductive material. In the top view, the metal-1 and metal-4 runners form a helix with each metal-4 runner interconnecting successive metal-1 runners. Similarly, the metal-2 and metal-3 runners form a helix with each metal-3 runner electrically interconnecting successive metal-2 runners. The resulting structure comprises a transformer, with an outer winding formed by the metal-1 and metal-4 runners and an inner winding formed by the metal-2 and metal-3 runners.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention can be more easily understood and the further advantages and uses thereof more readily apparent, when considered in view of the detailed description of the invention and the following figures in which:

[0014]FIGS. 1 through 11 illustrate, in cross-section, a transformer structure according to embodiments of the present invention during sequential fabrication steps; and

[0015]FIGS. 12, 13 and 14 illustrate top views of transformer structures according to several embodiments of the present invention.

[0016] In accordance with common practice, the various described device features are not drawn to scale, but are drawn to emphasize specific features relevant to the invention. Reference characters denote like elements throughout the figures and text.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The description of the method for forming a transformer described below is generally directed to the formation of the elements associated with a single winding of the outer and the inner transformer coils. It will be apparent to those skilled in the art that a plurality of such windings are being simultaneously formed in the integrated circuit substrate. However, in the description below it is also frequently necessary to refer to interconnections between successive windings.

[0018] The preferred process according to the present invention begins as shown in FIG. 1 where a plurality of insulating layers are formed over an existing integrated circuit substrate, which conventionally includes a plurality of active elements. Typically, at this point in the conventional fabrication process, no metal interconnect layers have been formed interconnecting active device regions; only the vias or windows for gaining access to the active device regions have been formed. A barrier layer 20 overlies the surface of the semiconductor substrate and is preferably formed of tantalum, tantalum-nitride, titanium or titanium-nitride. Next an insulating layer 22 of a relatively low dielectric constant material is formed over the barrier layer 20. Low dielectric silicon dioxide, black diamond and coral are suitable candidates for the insulating layer 22. The relative dielectric constant for silicon dioxide is about 3.9. Thus a low relative dielectric constant is generally considered to be less than about 3.0. A low dielectric constant material reduces inter-layer capacitance and therefore potential cross-talk between signals carried on the metal interconnects adjacent the dielectric layers. The barrier layer 20 and the insulating layer 22 can be formed by chemical vapor deposition.

[0019] In a preferred embodiment, a layer 24 overlying the insulating layer 22 comprises a hard mask of silicon dioxide. To etch a layer or layers below a hard mask, photoresist material is applied over the hard mask, the photoresist is patterned and then the pattern is transferred from the photo resist to the hard mask. The photoresist is removed and the etching steps are carried out using the hard mask pattern. This process advantageously offers better dimensional control of the etched features. In lieu of a hard mask, conventional photoresist patterning and etching steps can be utilized. In either case, as shown in FIG. 2, a window or trench 30 is formed in the barrier layer 20, the insulating layer 22 and the hard mask layer 24, by the use of suitable etchants. In the top view, the trench 30 is circular or elliptical. Generally, patterning and etching steps do not allow formation of sharp-cornered structural shapes, and thus windows and trenches, when viewed from the top, are typically circular, elliptical, or have relatively straight edges and rounded corners between the edges.

[0020] Turning to FIG. 3, a barrier and seed layer 32 is deposited. Typically, this is accomplished in two steps. First a barrier material is sputtered into the trench 30. Tantalum, tantalum-nitride, titanium and titanium-nitride are candidate materials for the barrier layer. Next, a thin copper seed layer is deposited, preferably by sputtering. The seed layer is required as a starting layer for the electroplated copper. Both the barrier material and the seed material of the barrier and seed layer 32 can also be deposited by conventional chemical vapor deposition and electroplating processes. A metal-1 runner layer 34 is now formed, preferably by electroplating copper. Electroplating is especially advantageous because it can be performed at a low temperature at a relatively low cost. The low temperature deposition feature is advantageous as it avoids changes in the dopant profiles. The substrate is then chemically-mechanically polished to remove the electroplated copper from all regions except within the metal-1 runner 34. This process for depositing copper layers in the insulating layers is known as the damascene process. It offers superior dimensional control because it eliminates the variations introduced in a conventional metal pattern and etch process where the vias and the interconnects are formed in two separate steps.

[0021] In certain circuit configurations it may be necessary to connect the metal-1 runner 34 to underlying regions in the substrate,. For instance, one end of the metal-1 runner can serve as a transformer terminal for connection to another component in the circuit. This can be accomplished by a dual damascene process by first forming a via opening connecting one end of the metal-1 runner to an underlying device region. The second step forms the trench 30, and the third step simultaneously fills the via opening and the trench 30 to form a conductive via and the metal-1 runner 34. Thus by this technique the metal-1 runner is connected to the underlying device region.

[0022] Although the present invention is described and shown in the Figures with the bottom segment of the outer winding formed in the metal-1 layer (and the top winding segment thereof formed in the metal-4 layer), and thus the reference to the metal-1 runner, the teachings of the present invention can be applied to form the bottom winding segment in a metal layer above metal layer 1. For example, the bottom winding segment of the outer winding can be formed from a metal-2 runner and the top winding segment can be formed from a metal-5 runner. Similarly, the inner winding top and bottom metal segments can be formed in various layers, so long as they are formed between the top and bottom winding segments of the outer winding.

[0023] As shown in FIG. 4, a second four-layer stack is now formed over the metal-1 runner 34 and the adjacent regions of the layers 20, 22 and 24. The bottom layer in the four-layer stack comprises a barrier layer 40 (preferably of titanium-nitride) as shown. An insulating layer 42 preferably having a relatively low dielectric constant is formed over the barrier layer 40 and comprises low dielectric constant silicon-dioxide, black diamond or coral. The use of a low dielectric constant material is advantageous to reduce inter-layer capacitance and thus inter-layer cross-talk, but it is not required that the insulating layer 42 comprise a low-dielectric material. An etch stop layer 48, formed of, for example, silicon-nitride, is formed over the insulating layer 42. Another insulating layer 50, preferably having a low dielectric constant, is formed over the etch stop layer 48. A hard mask layer 52 is formed over the insulating layer 50. As discussed above, conventional photoresist, masking and patterning processes can be used in lieu of the hard-mask layer 52.

[0024] Turning to FIG. 5, a masking step employing the hard mask layer 52 defines the areas where second-level via openings 60 and 62 are to be formed. Using the defined pattern, the second-level via openings 60 and 62 are then etched downwardly to the barrier layer 40. An additional etch step is then employed to remove the barrier layer 40 from the base of the via openings 60 and 62. At this point in the fabrication process there may be other areas of the integrated circuit that also require via openings at this level and therefore these can be formed simultaneously with the via openings 60 and 62. In general, this is the case when any of the layers associated with the transformer are formed; via openings and trenches required in other regions of the integrated circuit can be simultaneously formed. Thus the method of the present invention does not add any additional masking steps to the integrated circuit formation process. It is only necessary to define additional regions for the transformer structures within the masks used to form vias and interconnects throughout the integrated circuit.

[0025] As further illustrated in FIG. 6, a trench 63 is formed, extending downwardly to the etch stop layer 48. In a preferred embodiment, to effectively stop the etch process at the etch stop layer 48, the etching process is monitored to analyze the byproducts that are etched from the material. In this case, when the material of the etch stop 48 is detected, the etch process is terminated. As a result, the trench 63 extends downwardly through the mask layer 52 and the insulating layer 50, terminating within the etch stop layer 48. As can be seen for the finished structure of FIG. 12, preferably the trench 63 is offset from the vertical plane of the metal-1 runner 34, such that the vertical plane of the trench 63 is behind the vertical plane of the metal-1 runner 34.

[0026] A barrier and seed layer 64 is deposited within the via openings 60 and 62 and the trench 63. The process and materials are identical to those discussed in conjunction with the barrier and seed layer 32 of FIG. 3. As illustrated in FIG. 7, copper is then preferably electroplated within the via openings 60 and 62 and the trench 63, followed by a chemical and mechanical polishing step to planarize the top surface. At this point, the two copper regions in the lower portion of the via openings 60 and 62 are referred to as conductive vias 65 and 66. The copper material in the upper regions (i.e., in the same horizontal plane as the trench 63) of the via openings 60 and 62 is referred to as metal-2 via layers 67 and 68, respectively. The copper material in the trench 63 is referred to as a metal-2 runner 69.

[0027] As shown in FIG. 8, a multi-layer stack is formed over the existing layers, where the material of the individual layers is preferably identical to the materials used in the multi-layer stack discussed in conjunction with FIG. 4. In particular, the layers formed sequentially include a barrier layer 70, an insulating layer 72 (preferably comprising material having a low dielectric constant), an etch stop layer 74, an insulating layer 76 (again preferably comprising a low dielectric constant material), and a hard mask layer 78.

[0028] As shown in FIG. 9, the hard mask layer 78 is patterned and etched to form four via openings. Two via openings 80 and 81 extend downwardly from the hard mask layer 78 to the top surface of the barrier layer 70 in substantial vertical alignment with the metal-2 via layers 67 and 68, respectively. Two additional via openings 82 and 83 extend downwardly from the hard mask layer 78 to the top surface of the barrier layer 70 in vertical alignment with end regions 84 and 85 of the metal-2 runner 69. Recall that the metal-2 runner is in a plane behind the metal-1 runner 34, thus the via openings 82 and 83 are in a vertical plane behind the via openings 80 and 81. In one embodiment the via openings 80, 81, 82 and 83 have the same size in the horizontal dimension of FIG. 9. The exposed barrier layer 70 at the bottom of the via openings 80, 81, 82 and 83 is then removed by an additional etch step. The hard mask layer 78 is again patterned and etched to form a trench 87 extending vertically from the hard mask layer 78 to the top surface of the etch stop layer 74. As can be seen from the top view of FIG. 12, the trench 87 is in a vertical plane that intersects the vertical plane of the metal-2 runner 69. Thus, the trench 87 extends rearward from the vertical plane of the metal-2 runner 69 for interconnecting two successive parallel metal-2 runners 69.

[0029] Barrier layers 90 are then applied to the interior surfaces of the four via openings 80, 81, 82 and 83 and the trench 87. Metal is then deposited or electroplated within the via openings 80, 81, 82 and 83 and the trench 87. Thus conductive vias 92 and 94 are formed within the via openings 80 and 81, respectively. The two conductive regions in the lower portion of the via openings 82 and 83 are referred to as conductive vias 96 and 98, respectively. The conductive material in the upper regions of the via openings 82 and 83 is referred to as metal-3 via layers 100 and 102, respectively. The conductive material in the trench 87 is referred to as a metal-3 runner 104. The resulting structure is illustrated in FIG. 10, but again note that the metal-3 runner 104 interconnects two successive metal-2 runners 69, which is not necessarily apparent from FIG. 10. According to the top view of FIG. 12, the end of the metal-3 runner 104 in electrical contact with the conductive via 98 and the metal-2 runner 69 in a first vertical plane, and the other end of the metal-3 runner 104 is in electrical contact with the conductive via 96 of the next rearward metal-2 runner 69, which is in a second vertical plane behind the first vertical plane.

[0030] To complete formation of the transformer of the present invention, another stack of insulating layers is disposed on the top surface of the FIG. 10 structure. As shown in FIG. 11, this stack of insulating layers comprises: a barrier layer 110, a dielectric layer 112 (preferably formed of a material having a relatively low dielectric constant) an etch stop layer 114 (preferably formed of silicon nitride) a dielectric layer 116 (preferably formed of a material having a relatively low dielectric constant) and a hard mask layer 118. A patterning and etching process, using a pattern masked into the hard mask layer 118, forms a pair of via openings extending downwardly from the hard mask 118 to the top surface of the barrier layer 110 and in substantially vertical alignment with the conductive vias 92 and 94. The exposed portions of the barrier layer 110 at the bottom of the pair of via openings is then removed. A second masking and etching step forms a trench extending between the pair of via openings and having a bottom surface adjacent the top surface of the etch stop layer 114. A barrier layer 120, shown in FIG. 11, is deposited on the interior surfaces of the pair of vias and the trench. Metal, preferably copper, is deposited to form conductive vias 122 and 124 and metal-4 via layers 126 and 128 aligned vertically therewith, respectively. An interconnecting metal-4 runner 130 is also formed, to interconnect two successive metal-1 runners via the two stacks of conductive vias shown in FIG. 11. Thus one end of the metal-4 runner 130 is in the same vertical plane as the conductive via 124 and the metal-4 via layer 128 connected to the metal-1 runner 34, and the other end of the metal-4 runner is connected to the metal-4 via layer 126 and the conductive via 122 connected to the next metal-1 runner 34 in the plurality of parallel metal-1 runners 34.

[0031] According to FIG. 11, the transformer appears as two concentric closed polygons (rectangles in FIG. 11) forming the outer and inner windings. A top view of the transformer constructed according to the teachings of the present invention, is illustrated in FIG. 12 and reveals the third dimension orientation of the various transformer components. Two successive parallel metal-1 runners 34 are interconnected by a diagonal metal-4 runner 126, via vertical conductive structures 130 and 132. The vertical conductive structure 132 comprises the conductive via 65, the metal-2 via layer 67, the conductive via 92, the conductive via 122 and the metal-4 via layer 126. The vertical conductive structure 130 comprises the conductive via 66, the metal-2 via layer 68, the conductive via 94, the conductive via 124 and the metal-4 via layer 128. Similarly, two successive parallel metal-2 runners 69 are interconnected by a diagonal metal-3 runner 104 via vertical conductive structures 136 and 138. The vertical conductive structure 138 comprises the conductive via 96 and the metal-3 via layer 100. The vertical conductive structure 136 comprises the conductive via 98 and the metal-3 via layer 102.

[0032] The plurality of metal-1 runners 34 and the plurality of metal-4 runners 126 can take on various other orientations and interconnection configurations. For instance, the angle between each metal-1 runner 34 and the metal-4 runner 126 can be made greater than or equal to 90° to produce a zig-zag pattern in the top view. See FIG. 13. Alternatively, both the metal-1 runners 34 and the metal-4 runners 126 can be L-shaped and interconnected such that the short leg on one runner connects to the long leg of the next runner. See FIG. 14. Typically, the transformer secondary windings, comprising the metal-2 runner 69 and the metal-3 runner 104, have the same shape and orientation as the primary windings. See FIGS. 12, 13 and 14.

[0033] The turns ratio between the outer and inner windings (either of which can operate as the primary winding while the other operates as the secondary winding) of the transformer can be modified by changing the distance between successive metal-2 runners 69 relative to the distance between successive metal 1-runners 34, so that greater or fewer number of coils or turns comprising the metal-2 and metal-3 runners 69 and 104 are positioned between successive metal-1 runners 34.

[0034] Although the Figures and description herein illustrate placement of the bottom and top metal layers of the outer winding in the metal-1 and metal-4 layers of the integrated circuit, the inventive features of the present invention can be applied such that the transformer structure spans other metal layers, for example, the bottom segment of the outer windings can be placed within the metal-2 layer and the top segment within the metal-5 layer. The inner windings similarly span any number of metal layers between the metal layers spanned by the outer winding. Other embodiments where different metal layers are spanned are considered within the scope of the present invention.

[0035] Advantageously, the multi-layer transformer formed according to the teachings of the present invention is compatible with conventional CMOS backflow (i.e., interconnect) processing and does not require any additional masking steps during the process of fabricating the CMOS devices. Because the conductive structures are formed of copper, the resulting conductor has relatively lower resistance than those formed with aluminum and thus a higher Q. The inner winding is completely enclosed within the outer winding, resulting in a relatively high coupling factor. As illustrated by the processing steps discussed above, the transformer is highly integratable either on-chip with other active elements or as part of a multi-module device constructed on a common substrate. Although the two windings are conventionally designed for simultaneous use to provide transformer action, they can be used independently as inductors.

[0036] Although formation of the outer and inner windings of the transformer according to the present invention has been described using a damascene process, the invention is not limited thereto. The transformer windings can also be formed using conventional metal deposition and etch steps wherein the metal layers forming the top and bottom winding segments are interconnected by vertical vias spanning at least three metal layers, i.e., at least one metal layer is not used to form either a top or a bottom winding segment.

[0037] An architecture and process have been described as useful for forming a thin film multi-layer high Q transformers on a semiconductor substrate. While specific applications of the invention have been illustrated, the principals disclosed herein provide a basis for practicing the invention in a variety of ways and in a variety of circuit structures. Numerous variations are possible within the scope of the invention, including the use of any two metal layers to form the transformer windings. The invention is limited only by the claims that follow. 

What is claimed is:
 1. A method of forming an integrated circuit structure comprising an inner winding and an outer winding, wherein the method comprises: forming a semiconductor substrate having an upper surface therein; wherein forming the outer winding comprises the steps of: forming a first upper and a first lower conductor layer over the upper surface; interconnecting the first upper and the first lower conductor layers to form a helical inductor structure; wherein there is at least one unconnected conductor layer between the first upper and the first lower conductor layers; wherein forming the inner winding comprises the steps of: forming a second upper and a second lower conductor layer between the first upper and the first lower conductor layers; and interconnecting the second upper and the second lower conductor layers to form a helical inductor structure.
 2. The method of claim 1 wherein the first upper and the first lower conductor layers each comprise a plurality of first upper and first lower conductive strips, and wherein the plurality of first upper and first lower conductive strips are in intersecting vertical planes, and wherein a first end of a first one of the plurality of first upper conductive strips overlies a first end of a first one of the plurality of first lower conductive strips, and wherein a second end of the first one of the plurality of first upper conductive strips overlies a second end of a second one of the plurality of first lower conductive strips, further comprising forming a first substantially vertical conductive via for interconnecting the first end of the first one of the plurality of first upper conductive strips and the first end of the first one of the plurality of first lower conductive strips, and further comprising forming a second substantially vertical conductive via for interconnecting the second end of the first one of the plurality of first upper conductive strips and the second end of the second one of the plurality of first lower conductive strips.
 3. The method of claim 1 wherein the integrated circuit structure comprises a plurality of metal layers, and wherein the first lower conductor layer is formed in one of the plurality of metal layers, and wherein the first upper conductor layer is formed in a metal layer at least three metal layers above the first conductor layer.
 4. The method of claim 3 wherein an end of the first lower conductor layer is interconnected to an end of an overlying first upper conductor layer with at least two vertically aligned conductive vias therebetween.
 5. The method of claim 1 wherein the second upper and the second lower conductor layers each comprise a plurality of second upper and second lower conductive strips, and wherein the plurality of second upper and second lower conductive strips are in intersecting vertical planes, and wherein a first end of a first one of the plurality of second upper conductive strips overlies a first end of a first one of the plurality of second lower conductive strips, and wherein a second end of the first one of the plurality of second upper conductive strips overlies a second end of a second one of the plurality of second lower conductive strips, further comprising forming a first substantially vertical conductive via for interconnecting the first end of the first one of the plurality of second upper conductive strip s and the first end of the first one of the plurality of second lower conductive strips, and further comprising forming a second substantially vertical conductive via for interconnecting the second end of the first one of the plurality of second upper conductive strips and the second end of the second one of the plurality of second lower conductive strips.
 6. The method of claim 1 wherein the integrated circuit structure comprises a plurality of metal layers, and wherein the second lower conductor layer is formed in at least the second metal layer of the integrated circuit structure, and wherein the second upper conductor layer is formed at least one metal layer above the second lower conductor layer.
 7. The method of claim 6 wherein an end of the second lower conductor layer is interconnected to an overlying end of the second upper conductor layer with at least one conductive via extending therebetween.
 8. A method for forming a multi-layer transformer within a semiconductor substrate, comprising: providing a semiconductor substrate; forming a first insulating layer over the semiconductor substrate; forming a plurality of parallel first level metal runners in the first insulating layer; forming a second insulating layer over the first insulating layer; forming a plurality of first and second conductive vias within the second insulating layer, wherein at the bottom end thereof, each one of the plurality of first and second conductive vias is in electrical contact with a first end and a second end, respectively, of each one of the plurality of first level metal runners; forming a plurality of second level metal runners in an upper portion of the second insulating layer; forming a third insulating layer over the second insulating layer; forming a plurality of third, fourth, fifth and sixth conductive vias within the third insulating layer, wherein each one of the plurality of third and fourth conductive vias is in substantially vertical alignment and in electrical contact with one of the plurality of first and second conductive vias, respectively, and wherein each one of the plurality of fifth and sixth conductive vias is in electrical contact with a first end segment and a second end segment, respectively, of each one of the plurality of second level metal runners; forming a plurality of parallel third level metal runners interconnecting fifth and sixth conductive vias at the upper end thereof, wherein the plurality of third level metal runners are formed in an upper portion of the third insulating layer; forming a fourth insulating layer over the third insulating layer; forming a plurality of seventh and eighth conductive vias within the fourth insulating layer, wherein each one of the plurality of seventh and eighth conductive vias is in substantially vertical alignment and in electrical contact with one of the plurality of third and fourth conductive vias, respectively; forming a plurality of parallel fourth level metal runners interconnecting seventh and eighth conductive vias at the upper end thereof, wherein the plurality of fourth level metal runners are formed in an upper portion of the fourth insulating layer; wherein each one the plurality of fourth level metal runners intersects successive first level metal runners, and wherein a first end of a fourth level metal runners is electrically connected to the first end of a first first level metal runner by way of the first, third and seventh conductive vias, and wherein a second end of the fourth level metal runner is electrically connected to the second end of a second first level metal runner by way of the second, fourth and eighth conductive vias; and wherein each one the plurality of third level metal runners interconnects successive second level metal runners, and wherein a first end of a third level metal runner is electrically connected to the first end of a first second level metal runner by way of the fifth conductive via, and wherein a second end segment of the third level metal runner is electrically connected to the second end segment of second second level metal runner by way of the sixth conductive vias.
 9. The method of claim 8 wherein the semiconductor substrate comprises additional insulating layers above the fourth insulating layer, and wherein the plurality of first level metal runners are formed in or above the first insulating layer, and wherein the plurality of fourth level metal runners are formed at least three insulating layers above the first level metal runner.
 10. The method of claim 8 wherein the semiconductor substrate comprises additional insulating layers above the fourth insulating layer, and wherein the plurality of second level metal runners are formed in or above the second insulating layer, and wherein the plurality of third level metal runners are formed at least one insulating layer above the second level metal runner.
 11. A method for forming at least two multi-layer concentric coils within a semiconductor substrate, comprising: providing a semiconductor substrate; forming a first stack of layers over the semiconductor substrate; forming a plurality of substantially parallel first trenches along a concentric axis and within the first stack of layers; forming conductive material within each one of the plurality of first trenches; to form a plurality of first level metal runners; forming a second stack of layers overlying the first stack of layers; forming a plurality of first and second vias openings within the second stack of layers, wherein each one of the plurality of first and second via openings is in contact with a first end and a second end, respectively, of each one of the plurality of first level metal runners; forming a plurality of substantially parallel second trenches within a predetermined number of layers of the second stack of layers and displaced vertically from the plurality of first trenches, wherein the horizontal plane of the plurality of second trenches is substantially parallel to the horizontal plane of the plurality of first trenches; forming conductive material within the plurality of first and second via openings to form first and second conductive vias and within the plurality of second trenches to form a plurality of second level metal runners; forming a third stack of layers overlying the second stack of layers; forming a like plurality of third, fourth, fifth and sixth via openings within the third stack of layers, wherein each one of the plurality of third and fourth via openings is vertically aligned with one of the plurality of first and second conductive vias, respectively, and wherein each one of the plurality of fifth and sixth via openings is vertically aligned with a first end and a second end, respectively, of each one of the plurality of second level metal runners; forming a plurality of substantially parallel third trenches within a predetermined number of layers of the third stack of layers, wherein each one of the plurality of third trenches interconnects a sixth via opening aligned with one of the plurality of second level metal runners with a fifth via opening aligned with the next parallel one of the plurality of second level metal runners; forming conductive material within the plurality of third, fourth, fifth and sixth via openings and the plurality of third trenches to form a plurality of third and fourth conductive vias, and to form fifth and sixth conductive vias and a plurality of third level metal runners in electrical contact therewith; forming a fourth stack of layers overlying the third stack of layers; forming a like plurality of seventh and eighth via openings within the fourth stack of layers, wherein each one of the plurality of seventh and eighth via openings is vertically aligned with one of the plurality of third and fourth conductive vias, respectively; forming a like plurality of substantially parallel fourth trenches within a predetermined number of layers of the fourth stack of layers, wherein each one of the plurality of fourth trenches interconnects an eighth via opening aligned with one of the plurality of first level metal runners with a seventh via opening aligned with the next parallel one of the plurality of first level metal runners, and wherein the horizontal plane of the plurality of fourth trenches is parallel to the horizontal plane of the plurality of first level metal runners; forming conductive material within the plurality of seventh and eighth via openings and the plurality of third trenches to form a plurality of seventh and eighth conductive vias and a plurality of fourth level metal runners in electrical contact therewith; wherein one of the plurality of fourth level metal runners is electrically connected between two successive first level metal runners by way of one of the plurality of the first, third and seventh conductive vias at a first end of the fourth level metal runner and by way of one of the plurality of the second, fourth and eighth conductive vias at a second end of the fourth level metal runner; and wherein one of the plurality of third level metal runners is electrically connected between two successive second level metal runners by way of one of the plurality of the fifth conductive vias at a first end of the third level metal runner and by way of one of the plurality of the sixth conductive vias at a second end of the third level metal runner
 12. The method of claim 11 wherein the first stack comprises a bottom barrier layer and an dielectric layer.
 13. The method of claim 12 wherein the material of the barrier layer is selected from among tantalum, tantalum-nitride, titanium and titanium-nitride.
 14. The method of claim 12 wherein the material of the dielectric layer comprises a material having a relative dielectric constant of about less than 3.0.
 15. The method of claim 12 wherein the material of the dielectric layer comprises silicon dioxide.
 16. The method of claim 12 wherein the first stack further comprises a hard mask layer overlying the dielectric layer, and wherein the plurality of first trenches are formed by patterning and etching through the hard mask layer.
 17. The method of claim 11 wherein the plurality of first, second, third and fourth trenches and the plurality of first, second, third, fourth, fifth, sixth, seventh and eighth via openings are formed by disposing a photoresist layer on the underlying layers and patterning and etching through the photoresist material.
 18. The method of claim 11 wherein the step of forming the plurality of first level metal runners further comprises: forming a barrier layer along the interior surfaces of each one of the plurality of first trenches; forming a seed layer adjacent the barrier layer; electroplating metal in each one of the plurality of first trenches; and planarizing the top surface of the substrate.
 19. The method of claim 18 wherein the material of the barrier layer is selected from among tantalum, tantalum-nitride, titanium and titanium-nitride, and wherein the barrier layer is formed by chemical vapor deposition.
 20. The method of claim 18 wherein the material of the seed layer comprises copper, and wherein the seed layer is formed by chemical vapor deposition.
 21. The method of claim 18 wherein the metal comprises copper.
 22. The method of claim 11 wherein the second, third and fourth stacks of layers comprise: a bottom barrier layer; a first dielectric layer overlying the bottom barrier layer; an etch stop layer overlying the first dielectric layer; and a second dielectric layer overlying the etch stop layer.
 23. The method of claim 22 wherein the material of the bottom barrier layer is selected from among tantalum, tantalum-nitride, titanium and titanium-nitride.
 24. The method of claim 22 wherein the material of the first and the second dielectric layers comprises a material having a relative dielectric constant of less than about 3.0.
 25. The method of claim 22 wherein the material of the first and the second dielectric layers comprises silicon dioxide.
 26. The method of claim 22 wherein the second, third and fourth stacks further comprise a hard mask layer overlying the second dielectric layer, and wherein the plurality of second, third and fourth trenches and the plurality of first, second, third, fourth, fifth, sixth, seventh and eighth via openings are formed by patterning and etching through the hard mask layer.
 27. The method of claim 11 wherein the predetermined number of layers of the second, third and fourth stacks of layers comprises the second dielectric layer.
 28. The method of claim 11 wherein the step of forming the plurality of first, second, third and fourth conductive vias further comprises: forming a mask layer over the stack of layers in which the conductive via is to be formed; patterning and etching the mask layer to form a via opening; forming a barrier layer within the via opening; forming a seed layer over the barrier layer; electroplating metal in the via opening; and planarizing the top surface.
 29. The method of claim 28 wherein the material of the barrier layer is selected from among tantalum, tantalum-nitride, titanium and titanium-nitride, and wherein the barrier layer is formed by chemical vapor deposition.
 30. The method of claim 28 wherein the material of the seed layer comprises copper, and wherein the seed layer is formed by chemical vapor deposition.
 31. The method of claim 11 wherein the step of forming conductive material within the plurality of the fifth and the sixth via openings and the third trench further comprises: forming a barrier layer within each one of the plurality of fifth and sixth via openings and the third trench; forming a seed layer overlying the barrier layer; electroplating metal in each one of the plurality of fifth and sixth via openings and the third trench; and planarizing the top surface of the substrate.
 32. The method of claim 31 wherein the material of the barrier layer is selected from among tantalum, tantalum-nitride, titanium and titanium-nitride, and wherein the barrier layer is formed by chemical vapor deposition.
 33. The method of claim 31 wherein the material of the seed layer comprises copper, and wherein the seed layer is formed by chemical vapor deposition.
 34. The method of claim 11 wherein the step of forming conductive material within the plurality of the seventh and eighth via openings and the fourth trench further comprises: forming a barrier layer within each one of the plurality of seventh and eighth via openings and the fourth trench; forming a seed layer overlying the barrier layer; electroplating metal in each one of the plurality of seventh and eighth via openings and the fourth trench; and planarizing the top surface.
 35. The method of claim 34 wherein the material of the barrier layer is selected from among tantalum, tantalum-nitride, titanium and titanium-nitride, and wherein the barrier layer is formed by chemical vapor deposition.
 36. The method of claim 34 wherein the material of the seed layer comprises copper, and wherein the seed layer is formed by chemical vapor deposition.
 37. The method of claim 11 wherein each one of the plurality of first, second, third and fourth level metal runners comprises an L-shaped structure in a top view of the semiconductor substrate, and wherein each L-shaped structure comprises a short leg segment and a long leg segment.
 38. The method of claim 37 wherein a short leg segment of one of the plurality of first level metal runners is electrically connected to a long leg segment of an adjacent one of the plurality of fourth level metal runners via the second, fourth and eighth conductive vias, and wherein a short leg segment of one of the plurality of second level metal runners is electrically connected to a long leg segment of an adjacent one of the plurality of third level metal runners via the sixth conductive via.
 39. The method of claim 11 wherein the plane containing one of the plurality of first level metal runners and the plane containing one of the fourth level metal runners intersect at an acute angle, and wherein the plane containing one of the plurality of second level metal runners and the plane containing one of the third level metal runners intersect at an acute angle.
 40. The method of claim 11 wherein the plurality of interconnected first level metal runners and fourth level metal runners form a first helical structure having a non-zero inductance, and wherein the plurality of interconnected second level metal runners and third level metal runners form a second helical structure having a non-zero inductance, and wherein the magnetic fields produced by the first and the second helical structures produce transformer action.
 41. The method of claim 11 wherein the semiconductor substrate has a plurality of metal layers, and wherein the plurality of first level metal runners are formed within one of the plurality of metal layers, and wherein the plurality of fourth metal runners are formed at least three metal layers above the plurality of first level metal runners, and wherein the plurality of second level metal runners are formed within one of the plurality of metal layers between the plurality of the first and the fourth level metal runners, and wherein the plurality of third metal runners are formed at least one layer above the plurality of second level metal runners.
 42. An integrated circuit structure comprising: a semiconductor substrate; an outer coil; and an inner coil disposed within the interior of said outer coil; said outer core further comprising: a plurality of parallel first conductive strips overlying said semiconductor substrate; a first stack of one or more conductive vias in electrical connection with a first end of each one of the plurality of first conductive strips; a second stack of one or more conductive vias in electrical connection with a second end of each one of the plurality of first conductive strips; and a plurality of parallel second conductive strips having a first end in electrical connection with the uppermost via of the first stack of one or more conductive vias electrically connected to a first one of the plurality of first conductive strips, and a second end in electrical connection with the uppermost via of the second stack of one or more conductive vias electrically connected to a second one of the plurality of first conductive strips, such that a second conductive strip is disposed between and interconnects two successive first conductive strips; said inner core further comprising: a plurality of parallel third conductive strips overlying said semiconductor substrate; a third stack of one or more conductive vias in electrical connection with a first end of each one of the plurality of third conductive strips; a fourth stack of one or more conductive vias in electrical connection with a second end of each one of the plurality of third conductive strips; and a plurality of parallel fourth conductive strips having a first end in electrical connection with the uppermost via of the third stack of one or more conductive vias electrically connected to a first one of the plurality of third conductive strips, and a second end in electrical connection with the uppermost via of the fourth stack of one or more conductive vias electrically connected to a second one of the plurality of third conductive strips, such that a fourth conductive strip is disposed between and interconnects two successive third conductive strips.
 35. A multi-level integrated circuit structure, comprising: a semiconductor substrate having a plurality of insulating layers and a plurality of conductive layers therebetween; an outer winding; and an inner winding at least partially disposed within the interior of said outer winding; said outer winding further comprising: runner conductive portions; vertical conductive portions; wherein lower runner portions are formed in a lower conductive layer of the semiconductor substrate; wherein upper runner portions are formed at least three conductive layers above the lower runner portions; wherein two or more vertically aligned first via portions effect electrical connection between a first end of a first lower runner portion and an overlying first end of a first upper runner portion; wherein two or more vertically aligned second via portions effect electrical connection between a first end of a second lower runner portion and an overlying second end of the first upper runner portion; said outer winding further comprising: runner conductive portions; vertical conductive portions; wherein lower runner portions are formed in a lower conductive layer of the semiconductor substrate; wherein upper runner portions are formed at least one conductive layer above said lower runner portions; wherein two or more vertically aligned first via portions effect electrical connection between a first end of a first lower runner portion and an overlying first end of a first upper runner portion; wherein two or more vertically aligned second via portions effect electrical connection between a first end of a second lower runner portion and an overlying second end of said first upper runner portion. 